This invention relates to a vibration damper assembly for a Vibrating Structure Gyroscope Resonator suitable particularly, but not exclusively, for use with a substantially ring-like, substantially planar resonator.
It is known to build Vibrating Structure Gyroscopes using planar ring members as the resonant structure. The principles of operation and practical implementation of ring structures in vibrating structure gyroscopes are described in the present Applicants EP/UK-B-0461761.
The ring is typically excited into a cos 2.theta. resonance mode. This mode actually exists as a degenerate pair of modes at a mutual angle of 45.degree.. The vibration patterns for these modes are shown schematically in FIG. 1A and 1B. One of the modes is excited as the carrier mode as shown in FIG. 1A with the other used as the response mode as shown in FIG. 1B. The carrier mode vibration is typically maintained at a constant amplitude at the peak resonance frequency. When the gyro body is rotated, Coriolis forces couple energy into the response mode. The amplitude of motion of the response mode is directly proportional to the applied rotation rate.
A particular implementation of this technology uses a Nickel-Iron alloy (Nilo-45) planar metal ring as the resonator 1 as shown in FIG. 2. The outer diameter of the ring 1 is 22 mm with a rim width of 1 mm and a depth of 1.2 mm. The ring is attached to a central hub 2 by means of eight compliant legs 3 allowing substantially undamped motion of the ring 1. The ring is attached to a support pedestal (not shown) at the central hub 2 to leave the ring 1 and legs 3 freely suspended. The ring structure, together with location of the carrier mode drive elements 4 (to set the ring into oscillation) and pick-off elements (to detect the motion) around the ring are shown schematically in FIG. 2. The carrier mode drives 4 excite the carrier mode resonance at two anti-nodal positions (i.e. points of maximum radial motion). The carrier mode pick-offs 5 detect this motion at the remaining anti-nodal points. The response mode pick-offs 6 are located at two of the response mode anti-nodal positions. The signal level on these pick-offs will be directly proportional to the applied rate.
Many potential sensor applications require the gyroscope to operate under quite demanding environmental conditions. One such requirement is that the rate output noise should remain within specified limits in response to an applied vibration spectrum. The applied vibration spectrum for a typical automotive application is 10-800 Hz at 2 g rms. However, certain military applications specify significantly higher vibration levels over wider spectral ranges.
The principle effect of an applied vibration will be to cause motion of the resonator with respect to the gyro body. As the pick-off modules are rigidly attached to the gyro body this will inevitably cause a corresponding relative motion between the resonator and pick-offs. The sensor pick-offs 5, 6 are of capacitive design with the pick-off plate and the circumferential edge of the ring 1 forming the two capacitor plates. The ring 1 is maintained at high voltage with respect to the pick-off plates. Any change in the pick-off to ring gap will give rise to a signal. Applied vibration in the plane of the ring 1 will modulate the capacitor plate gap at the input vibration frequency. The pick-offs 5, 6 are arranged in pairs with the signals from pick-offs positioned diametrically opposite one another across the ring being summed at the input to the signal processing electronics. These signals will be in anti-phase and will thus tend to cancel one another provided that the pick-off gains are well matched.
Vibration along the axis normal to the plane of the ring 1 will cause the rim to move with respect to the pick-off plate 5, 6, as shown in cross-section in FIG. 3. The net effect of this motion is also to modulate the pick-off to ring gap. the phase of the signals on the pick-off pair will be identical for this motion of the ring and will therefore sum together. Furthermore, if the pick-off plate and ring are not aligned precisely parallel this will also cause the gap size to change generating a vibration induced error signal.
Any relative motion of the resonator 1 with respect to the pick-offs 5, 6 will generate a signal on the pick-offs. The scheme for the derivation of the rate signal in the sensor means that only noise inputs around the ring resonance frequency, within the gyroscope bandwidth, pass through to the rate output. This is shown schematically in FIGS. 4A to 4F. The ring is vibrated at the cos 2.theta. resonance frequency which provides the carrier signal at which demodulation takes place (FIG. 4A). A rate input (FIG. 4B) modulates the carrier giving sidebands on the pick-off signal output as shown in FIG. 4C. The vibration induced noise will appear at the baseband frequency. In the absence of any filtering the gyroscope output will appear as in FIG. 4D. This is filtered by the effective system bandwidth (FIG. 4E) to give the observed rate output shown in FIG. 4F.
For the known gyroscope described here the carrier frequency is at 5 kHz. This is well above any applied vibration input band and thus the gyroscope is inherently insensitive to vibration under normal circumstances. However, at extreme levels of vibration the amplitude of the ring motion may become sufficiently large to saturate the pick-off amplifier output at particular times in the vibration cycle. This intermittent saturation will cause instability in the gyro control loops with a consequent degradation in noise performance.
The in-plane vibration modes of the resonator 1 described are all well above the input vibration range. The planar nature of the resonator 1 results in the out-of-plane stiffness being significantly lower than the in-plane, however. The out-of-plane mode frequencies thus occur at lower frequencies with the lowest being the bending mode, as shown in FIG. 3, which occurs at approximately 2 kHz. This vibration mode may be excited directly by external vibration. The high Quality Factor of the ring 1 (approximately 2000) may result in a large amplitude of motion in this mode. In the limit, extremely high levels of applied vibration may result in mechanical failure of the resonator 1.
The sensor rate output for this gyroscope is within the specified operating limits for typical civil applications vibration specification. Under some of the more demanding military specifications, saturation of the pick-off output due to the large amplitude of motion in the out-of-plane bending mode causes the noise level to rise to unacceptably high levels.
There is thus a requirement for an improved gyroscope capable of operating in these harsh vibration environments.
A known means of imp roving the vibration tolerance of a gyroscopic device is to fix it onto an anti-vibration mount. This restricts the g-level to which it is exposed by absorbing some of the relative motion in the external mount. A problem with this solution is that the mount needs to be large enough to accommodate the entire mass of the gyroscope. This in creases the over all volume occupied by the sensor. For military applications, in particular, this is undesirable as the space envelope available for the sensor pack may be restricted. External anti-vibration mounts typically damp vibration inputs along every axis. For rapid angular applied rate not being applied instantaneously to the gyroscope and will consequently restrict its bandwidth. Any motion of the external mount will also cause a misalignment of the sensor axis which is again undesirable.
There is thus a need for a generally improved vibration damper assembly for a Vibrating Structure Gyroscope and for a vibrating structure gyroscope incorporating such a vibration damper assembly.